Method and device for making a molten film of metal more uniform

ABSTRACT

The invention relates to a method for homogenizing a molten metal film, more particularly a steel film, by means of thin strip casting. According to the invention, the melt applied to a rotating strip is of a similar thickness to and possesses the same qualities wherever possible as the width of the strip. In order to homogenize the width of the strip, forces possessing a component perpendicular to the direction of conveyance of the strip are introduced, whereby homogenization of the profile of the molten metal film occurs.

DESCRIPTION

[0001] The invention relates to a method for making a molten film of metal, in particular a steel film, more uniform, in accordance with the preamble of patent claim 1, and to a device for carrying out the method.

[0002] The invention can be employed wherever a molten film of metal, in particular of steel, is applied to a substrate, in particular to a revolving conveyor belt, in molten form and its thickness and properties are to be as uniform as possible over the width of the strip.

[0003] During the strip casting of metal, in particular of steel, the cast thickness of the strip can to a large extent be selected optimally according to the required thickness during finish rolling and for the necessary hot forming to achieve sufficient materials properties. It is known to cool the molten metal using suitable methods and devices in such a way that the surface of the liquid strand of metal is cooled uniformly by contact with an inert gas.

[0004] DE 44 07 873 C2 has described a method and a device for cooling molten steel, in which nozzles are directed onto the surface of the steel strand at an angle of between 0 and 50° in the direction of casting, with the result that the steel surface is cooled uniformly and in a controlled manner. This makes it possible to avoid any scaling and to achieve controlled dissipation of heat, with the result that the surface tension is influenced in a controlled way and the desired quality of the steel strand or steel strip is achieved. However, a constant thickness also remains important to the quality of a strip made from steel with a view to achieving uniform materials properties over the width of the strip, and this cannot readily be achieved simply by applying the molten steel to the conveyor belt.

[0005] Therefore, the object of the invention is to improve the prior art in such a way that it becomes possible to alter a film of molten metal before and after it comes into contact with the conveyor belt, so that it has a uniform thickness with uniform materials properties over its width.

[0006] This object is achieved by means of the distinguishing part of patent claims 1 and 15.

[0007] The subclaims represent advantageous configurations of the invention.

[0008] To make the film of metal applied to the casting belt more uniform over its width, the solution according to the invention envisages forces being introduced, making the molten metal more uniform.

[0009] For the invention, it is advantageous for these forces to be introduced into the film of metal across the width of the strip in the opposite direction to the direction in which it is conveyed. For this purpose, the molten material flowing onto the conveyor belt should be decelerated by the action of the forces. If the molten film is flowing more quickly than the conveyor belt, the cross section which is taken up by the molten material is smaller than the cross section of the molten film moving synchronously with the conveyor belt (desired cross section). An insufficiently filled cross section of this nature represents a drawback. Decelerating and building up the molten material leads to the cross section being filled up uniformly. Excessive deceleration and an oversized molten film is to be avoided. Unlike in DE 44 07 873 C2, it is the geometric uniformity, even if it is achieved by means of a gas stream, rather than the cooling which is the principal factor. Accordingly, there are significant different features for the gas flow. Furthermore, force components which act perpendicularly to the surface assist with making the cross section more uniform.

[0010] It is advantageous for these forces to be applied oppositely to the direction in which the strip is conveyed by a gas stream directed onto the strip. Suitable gases are inert gases, such as argon or nitrogen, if appropriate with the addition of reducing components, for example H₂, CO, or oxidizing components which have an effect on the surface tension, such as O₂, CO₂.

[0011] Furthermore, it is advantageous for the gas to be applied to the film of metal at equal distances. This can be achieved by a row of nozzles which are arranged next to one another and are operated in such a way that the volumetric flow rate of gas flowing out exerts a force on the surface of the film of liquid metal. This force leads to the gas jets penetrating into the metal film to an extent of at least 50% of the thickness of the metal film. The intensity of each gas jet must be such that the liquid metal is prevented from splashing up and dispersion of gas bubbles into the molten material is avoided.

[0012] Furthermore, it is advantageous for gas nozzles to be arranged next to and behind one another, so that they are, as it were, in the shape of a rake. As a result, the film of liquid metal which is being conveyed in the opposite direction to that in which the gas flows out is treated by the emerging gas jets as if by a rake, with the result that the molten material is decelerated and made more uniform over the width of the strip. It is particularly advantageous for two or more rakes to be arranged one behind the other, in each case offset, acting in the same way as a Pascal's triangle. The result is that the thickness of the strip is as uniform as possible over its width and the materials properties of the strip are as uniform as possible over the width.

[0013] Furthermore, it is advantageous for the nozzles to be arranged at an angle which is such that the gas stream impinges on the surface of the molten film oppositely to the direction of flow of the cast strip, at an angle of between 10 and 80° to the vertical. To control the thickness of the cast strip, it is furthermore advantageous for the thickness of the molten film to be determined by suitable sensors after it has been applied and for the gas flow emerging from the nozzles to be controlled by means of a suitable control device in such a way that this gas stream acts on the thickness of the strip over the width of the strip in a controlled manner.

[0014] Furthermore, it is advantageous for an agent which initiates solidification to be applied to the film of metal, in order to achieve advantageous solidification of the surface. For steel, for example, the solidification-initiating agent used is an oxidizing CO₂-containing gas which causes-decarburization of a thin surface layer of the molten film so that the solidification temperature can be raised above the actual temperature to such an extent that the solidification starts from the top side. The CO₂ content must be kept sufficiently low to ensure that there is no formation of slag.

[0015] Other solidification-initiating agents which may be used include a cooling and nucleating powder, for example metal powder, a liquid slag, a gas or a further liquid metal

[0016] The invention is explained in more detail below with reference to two figures and an exemplary embodiment.

[0017]FIGS. 1 and 2 show the situation with flow modifications. The gas jets 7 emerge from gas nozzles 3 with apertures with a diameter of 1 mm in two rows at offset positions, from a copper section 6 containing two chambers, of which one chamber serves to supply the gas and one chamber serves for water cooling of the copper section 6. These gas jets 7 impinge on the molten material flowing onto the conveyor belt 2 oppositely to the casting direction and at an angle of 30° with respect to the surface normal, and decelerate this molten material. Depending on the reduced mean speed, the cross section of flow is increased to the desired level. Furthermore, it is possible to make the molten material more uniform in the transverse direction, in order to achieve a uniform thickness profile, in the molten material which has built up between the feed point and the area of incidence of the gas. Overall, the effect of the gas stream in the described form can be compared to that of a rake for achieving a uniform distribution of material (“Pascal's argon rake”).

[0018] As an additional option, it is possible to use a corresponding argon rake in order to provide a uniform distribution of material as early as at the feed plane.

[0019] Furthermore, to make the film of metal 4 more uniform, it is advantageous for such argon rakes to oscillate transversely with respect to the flow of metal.

List of Reference Numerals Used

[0020]1 Metal feed

[0021]2 Conveyor belt

[0022]3 Gas nozzle

[0023]4 Film of metal

[0024]5 Point of incidence of the gas on the film of metal

[0025]6 Copper section

[0026]7 Gas jet 

1. A method for making a molten film of metal, steel, in particular a steel film, more uniform, by strip casting, in which the molten material which is applied to a revolving belt is to have a thickness and properties which are as uniform as possible over the width of the strip, wherein to make the strip more uniform across its width, forces are introduced into the metal film with a component which is perpendicular to the direction in which the strip is conveyed, which forces make the profile of the molten film of metal more uniform.
 2. The method as claimed in claim 1, wherein the forces are applied to the strip by means of a gas stream which is directed oppositely to the direction in which the strip is conveyed.
 3. The method as claimed in one of claims 1 to 2, wherein the gas is collected and recycled after it has impinged on the film of metal.
 4. The method as claimed in one of claims 1 to 3, wherein a reducing gas is used.
 5. The method as claimed in one of claims 1 to 4, wherein the gas used is an inert gas.
 6. The method as claimed in one of claims 1 to 5, wherein a gas which has an effect on the surface tension is used.
 7. The method as claimed in one of claims 1 to 6, wherein the gas is applied to the strip in the form of individual jets at regular intervals.
 8. The method as claimed in one of claims 1 to 7, wherein the gas is applied at elevated temperature.
 9. The method as claimed in one of claims 1 to 8, wherein the thickness is measured across the width of the strip, and gas streams are activated in a controlled manner on the basis of the signals from this measurement.
 10. The method as claimed in one of claims 1 to 9, wherein the gas stream impinges on the surface of the strip oppositely to the direction of flow of the cast strip, at an angle of between 0 and 80° with respect to the vertical.
 11. The method as claimed in one of claims 1 to 10, wherein the gas stream impinges on the surface of the cast metal strip at a speed which is such that an indentation of at least half the thickness of the cast metal strip is formed in the liquid metal at the point of incidence.
 12. The method as claimed in claim 11, wherein the row of nozzles oscillates transversely with respect to the direction of flow.
 13. The method as claimed in one of claims 1 to 12, wherein an agent which initiates solidification is applied to the metal film which has been made more uniform.
 14. The method as claimed in one of claims 1 to 13, wherein the means which initiates solidification applied to the metal film which has been made more uniform is gas.
 15. The method as claimed in one of claims 1 to 14, wherein the gas used is an oxidizing gas.
 16. A device for carrying out the method as claimed in claims 1 to 15, wherein at least one row of gas nozzles (3) is arranged across the width of the strip directed oppositely to the direction of flow of the conveyor belt (2).
 17. The device as claimed in claim 16, wherein a plurality of rows of gas nozzles (3) are arranged one behind the other across the width of the conveyor belt (2), so that a profile resembling a bed of nails is formed on the film of liquid metal (4).
 18. The device as claimed in one of claims 16 and 17, wherein the gas nozzles (3) in the rows are arranged offset with respect to one another.
 19. The device as claimed in one of claims 16 to 18, wherein thickness-measuring sensors are arranged across the width of the strip, between the gas nozzles (3) and the metal feed (1).
 20. The device as claimed in one of claims 16 to 19, wherein a control unit is arranged between the thickness-measuring sensors and the gas nozzles (3).
 21. The device as claimed in one of claims 16 to 20, wherein a single gas nozzle (3) is arranged across the strip width, which nozzle acts on the film of metal through a narrow, long slot across the width of the strip, in such a way that a type of wave is formed across the width of the strip.
 22. The device as claimed in one of claims 1 to 16, wherein a gas nozzle (3) with a multiplicity of gas jets is arranged across the width of the strip. 